Field of the Invention
The present invention is directed generally to communications connector systems and related structures.
Description of the Related Art
A transmission line may have a first end opposite a second end. The second end may be attached to a load and referred to as a “load” end. The first end may be connected to a signal source. If the transmission line has constant impedance along its length, the transmission line will not reflect signals.
However, the time-rate-of-change in electrical signals, by nature, creates changing and propagating electric and magnetic fields along a transmission line. The objective of a transmission line is to contain these fields and to deliver them from one point in space to another and minimize the affect of external fields on the integrity of signal transmission along the line.
There are several ways to contain these fields along what is commonly known by those of ordinary skill in the art as Transverse Electric and Magnetic (“TEM”) transmission lines, e.g., lines having electrical conductors along the length of the line.
One TEM transmission line structure having shielded wires is known as a coaxial line where one conductor is tubular and shares the same axis with a second “coaxial” conductor. The tubular conductor is commonly called a “shield” and the other conductor is called the “center conductor.” Any voltage field created by the center conductor is intercepted by the shield, and any magnetic field generated by the center conductor is cancelled by the return of the same current from the load end thereby containing the electric and magnetic fields along the line. Since each conductor in this coaxial transmission line is not treated equally, it is called an unbalanced transmission line.
Alternatively, another TEM type transmission line structure is a differential wire pair transmission line. With differential transmission lines, the electric and magnetic fields are approximately cancelled by identical conductors (e.g., wires) with exactly opposite signals that share nearly the same space. The electric and magnetic fields are thus mostly contained in or around the conductors and a nearly insignificant portion of each field escapes the region near these “paired” conductors. This is called a balanced, or differential, transmission line. At a cost, a shield can be added to this differential pair to contain the “nearly insignificant” leakage field to the point where it can become insignificant.
In the process of a transmission line guiding electrical energy from one point to another, the electrical energy is in the form of varying voltages and currents that relate to each other by means of the impedance, which may be characteristic of a transmission line. Just as “Ohm's Law” applies to Direct Current (“DC”), characteristic impedance applies to time variant signals by setting the ratio of voltage to current on an infinitely long transmission line. It is symbolized by “Zo” and expressed in units of “Ohms.” Ideally this would be a simple injection of a signal into the “source end” of the transmission line and, after some propagation delay, the same signal arrives at the “load end” of the transmission line. Changes in, or discontinuities of, the transmission line's impedance, however, may cause some of the signal to reflect back upon itself. As understood by those of ordinary skill in the art, such reflection is described by the reflection coefficient, which preferably is zero:
  Γ  =                    Z        L            -              Z        S                            Z        L            +              Z        S            The subscripted Z's above are load-side and source side-impedances.
These reflections can occur anywhere along the TEM transmission line. There are usually many reflections in a TEM line. Such reflections are created by imperfections in the transmission cable uniformity which may be caused by a variety of reasons including imperfections in the manufacturing process, “dimensional” damage, conductor termination at connectors or transmission between source/generator and load/receiver that is unmatched to the transmission line's characteristic impedance.
The reflections in TEM transmission lines of various delays, amplitudes and spectral energies combine to obscure the original forward propagating signal. To minimize signal reflections and maximize the delivery of an unadulterated signal along the TEM transmission line, the transmission line system must be terminated by connectors at both ends of the line that maintain impedances equal to the characteristic impedance of the transmission line.
Although there are specific formulae that designate the impedance for different transmission line configurations, fundamentally the formula below indicates the parameters affecting the impedance of transmission lines:
      Z    0    ∝                    L        UNIT_LENGTH                    C        UNIT_LENGTH            The above formula indicates that the transmission line impedance will be lower if unit-length capacitance (represented by variable “C”) increases, or vice versa. Unit-length inductance (represented by variable “L”) typically does not change because materials associated with transmission lines typically do not have magnetic permeability characteristics that are different from “free space” which would alter this baseline inductance.
However, common insulator/dielectric materials that may surround a transmission line do alter free space permittivity and may alter capacitance. Geometric distances between the two conductors of a transmission line are easily altered and such alteration may also alter capacitance as reflected in the following formula:
  C  =            ɛ      ⁢                          ⁢      A        d  The above formula indicates that for a small but constant area, the capacitance increases as distance (represented by variable “d”) decreases. The variable “∈” represents a permittivity constant for the material in the vicinity of the transmission line and increases with increasing capacitance.
In sum, for a given dielectric material, the distance between two conductors of a transmission line affects the characteristic impedance of the cable and, in turn, would also affect the reflection of the transmission line if the capacitance changes along the longitudinal distance of the transmission line.
Excluding manufacturing non-uniformities and cable damage, the typical cause of unwanted reflections in a transmission line system is the dielectric and dimensional disturbance caused by connections that interrupt the geometry of transmission line cabling. This occurs because the cable must be cut and disassembled, usually involving splaying of the shield and wire (or wires if differential), thus causing a disturbance to the dielectric and the conductor spacing.
Any shielding of the differential pair of a transmission line may also affect the capacitance between the two differential conductors of the pair thereby creating reflections as discussed above. Moreover, if such a shield is a metal foil, it will usually expand away from the wire or wire pairs, but may also be cut or torn irregularly at one or more points along the transmission line thereby creating non-uniformities and mismatches between the transmission line, its shield, and any shielding provided by the connectors to which the transmission line may be connected.
In the case of a coaxial transmission line, the shield is one of the two transmission line conductors. In the case of a differential pair, however, the conductive shield is typically positioned intermediate the differential pair conductors and the cable jacket that may act as a capacitive stepping-stone, or shunt, that profoundly affects the sum-total capacitance between the transmission line's conductor pair thereby affecting the impedance of the system in a connector termination zone.
Traditionally, the use of a single drain wire to ground transmission lines operating at lower operational bandwidths/frequencies sufficed for adequate performance of a shielded transmission line. At higher operational bandwidths/frequencies, however, where the foil ends and the drain wire continues, the drain wire simply introduces a constriction in the cable ground. The gap between the end of the foil and the shielded connector becomes an unwanted aperture at these wavelengths.
If the length of this “disrupted shield” impedance discontinuity is significantly shorter than the shortest wavelength transmitted by the differential transmission line, the impedance will essentially go undetected because the low-to-high reflection and the high-to-low reflection at each end of the short discontinuity will cancel each other. However, shielding effectiveness would be disrupted if the shield was deformed so as to uncover a portion of the transmission line wires it originally encompassed.
As bandwidth needs increase, frequencies transmitted increase, and the wavelengths become shorter. Reflections at either end of the impedance discontinuity are no longer close enough together to be near enough to 180 degrees (or PI radians) out of phase, thus the low-to-high reflection and the high-to-low reflection will not cancel one another sufficiently to go unnoticed. Therefore, the system becomes vulnerable to shorter and shorter discontinuities and more care needs to be taken.
Thus, a need exists for devices configured to minimize reflections attributable to a connector termination zone, including disturbances caused by cable shielding, and the process of assembling a connector onto the end of a transmission line. A need also exists to improve the effectiveness of cable shielding by improved continuity of the shield in the vicinity of the disturbance created by assembling the end of the cable to a connector. A need also exists to reduce the dependency on an inductive drain wire to ground the shielding of a cable. The present application provides these and other advantages as will be apparent from the following detailed description and accompanying figures.